Electrical Properties of Micro-Photodiode Arrays for Use as Artificial Retina Implant

Micro-photodiode arrays have been developed which are intended to eventually replace the function of degenerated photoreceptor cells in the retina. Electric current generated by tiny micro-photodiodes is delivered to the neuronal network in the retina via micro-electrodes. Since the coupling between electrode and tissue is capacitive of nature, only transient signals may be used for stimulation. Therefore, high capacitance of the interface between electrodes and tissue is an important prerequisite for efficient charge transfer. In addition, the electrical properties of the micro-photodiodes as are reflected in current/voltage traces have a profound influence on the charge delivery. For the first time the electrical properties of the entire system consisting of micro-photodiode array, electrode/electrolyte interface and tissue were studied. Our results indicate charged balanced operation of the device and the presence of a non-zero electrode polarization. The latter may be avoided, if an active current sink is employed in order to draw current to discharge the electrode capacitance.